Description
Within the XPS insulation boards range, Atlas Fundament 90 mm is the precision thickness for below-DPC plinth zones where the foundation insulation has to align flush with the graphite EPS course on the facade above. The board delivers a thermal conductivity of λ 0.038 W/mK in a closed-cell foundation-grade structure, measuring 600 × 1250 mm with rebated edges and covering 0.75 m² per board.
What Atlas Fundament 90 mm Does in a UK EWI System
Plinth-to-facade alignment is the deciding factor on most EWI retrofits, and Atlas Fundament 90 mm exists to solve it. At R 2.37 m²K/W and λ 0.038 W/mK in a closed-cell foundation board, the 90 mm depth matches the most common graphite EPS facade thickness above DPC — letting the plinth zone read as a continuous insulation plane with the elevation above, with no step at the damp-proof course line and no render-thickness compensation to engineer around.
Each board measures 600 × 1250 mm with rebated edges that lock together to form a continuous insulation plane without thermal bridging at the joints, so the moisture-resistant layer reads as a single membrane from one course to the next and meets the facade insulation flush at DPC.
Why Specifiers Choose Atlas Fundament 90 mm for UK Walls
- Flush plinth-to-facade alignment: 90 mm matches the common 90 mm graphite EPS facade thickness above DPC, eliminating the step that 80 mm or 100 mm boards would force into the render finish line.
- Strong below-DPC thermal performance: R 2.37 m²K/W sits between standard Part L specification and deep-wrap territory, delivering meaningful thermal continuity through the plinth zone.
- Closed-cell moisture protection: Permanent resistance to ground moisture and hydrostatic pressure keeps thermal performance constant for the life of the foundation.
- Rebated edges for joint integrity: Tight overlapping joints reduce the cold-bridge risk that flat-edge boards leave at every horizontal course.
- Freeze-thaw stable: Holds full R-value through repeated UK winter cycles in exposed plinth and below-grade positions.
Thermal Specifications — Atlas Fundament 90 mm
| Parameter | Value | Notes |
|---|---|---|
| Thermal conductivity (λ) | 0.038 W/mK | Declared value for foundation board |
| Thickness | 90 mm | Plinth-alignment specification |
| Thermal resistance (R) | 2.37 m²K/W | At declared λ × thickness |
| Reaction to fire | Per EN 13501-1 | Confirm via current DoP at order |
| Water absorption | Low (closed-cell) | Suitable for permanent ground contact |
Physical Specifications — Atlas Fundament 90 mm
| Parameter | Value |
|---|---|
| Board dimensions | 600 × 1250 mm |
| Coverage per board | 0.75 m² |
| Edge profile | Rebated (lap joint) |
| Compressive strength | Foundation-grade (confirm via TDS at order) |
| Boards per m² | 1.33 (add 5–10% cutting waste) |
Where Atlas Fundament 90 mm Performs Best — Wall Types & Build-Up
The 90 mm thickness is purpose-specified for one detailing decision: how the plinth meets the facade. On EWI retrofits and new-builds where the elevation above DPC will be insulated with 90 mm graphite EPS — the most common facade thickness for UK domestic projects — the foundation board needs to land on the same plane. The 90 mm Atlas Fundament delivers that flush transition without forcing the render line to step or the basecoat to compensate at the DPC junction.
- Plinth zones with 90 mm graphite EPS above DPC: Position from ground level up to the damp-proof course where the facade above is being insulated with matching-thickness graphite EPS. The plinth-to-facade joint reads as one continuous plane.
- Below-DPC zones on solid-wall retrofits: Use on Victorian and Edwardian retrofit projects where the facade EWI build-up has been set at 90 mm to balance thermal target against window-reveal geometry.
- Foundation wraps targeting above-standard U-values: Where the project pushes past standard Part L thresholds but stops short of Passive-leaning depths, 90 mm sits in the engineered middle ground.
- Basement perimeter on deeper retrofits: Apply across basement and cellar perimeters where the project specifies a thermal target above the typical 80 mm baseline.
How Atlas Fundament 90 mm Fits Into a Full EWI System
In a full external wall insulation build-up, Atlas Fundament 90 mm handles the plinth and below-DPC foundation zone, while the main facade above is finished with matching-thickness graphite EPS boards bonded with adhesive-and-basecoat and finished with mesh-reinforced render. The alignment story is the system story: when the plinth XPS lands on the same plane as the facade EPS, the basecoat and mesh wrap the transition without thickness compensation, and the render reads as one continuous finish from ground level to eaves.
Bond the 90 mm board with a specialist polyurethane adhesive such as Ceresit CT84 XPS Foam, which cures fast in the cool, damp conditions typical of UK ground-level work. Above ground — particularly on exposed plinth runs — add mechanical fixings from the EWI fixing accessories range at 6–8 plugs per m², stepping up to 8 plugs per m² on corners and edges.
How Atlas Fundament 90 mm Compares to Sibling Foundation Boards
| Board | Thickness | R-Value (m²K/W) | Best For |
|---|---|---|---|
| Atlas Fundament 50 mm | 50 mm | 1.32 | Floor insulation, shallow foundation wrap |
| Atlas Fundament 80 mm | 80 mm | 2.11 | Standard foundation, basement walls, Part L target |
| Atlas Fundament 90 mm | 90 mm | 2.37 | Below-DPC plinth flush with 90 mm facade EPS |
| Atlas Fundament 100 mm | 100 mm | 2.63 | Deep foundation wrap, highest thermal target |
For projects weighing closed-cell foundation insulation against graphite EPS for the main facade, the XPS vs EPS comparison guide walks through where each material wins across the build-up.
How Atlas Fundament 90 mm Performs in UK Weather — Moisture, Frost, Wind Loads
The closed-cell structure keeps long-term water absorption at near-zero, so freeze-thaw cycling across a typical UK winter does not progressively reduce R-value the way it can on open-cell boards exposed to ground moisture. In splash-zone positions at the plinth, the 90 mm board holds its thermal performance through saturation, drying, and re-saturation cycles for decades — critical for the zone where driven rain, ground splash, and capillary moisture combine throughout a UK winter.
Wind-load exposure is a meaningful factor above grade. The 90 mm face area catches more uplift than the 80 mm board, so the mechanical fixing layout should be set to the project-specific pattern rather than a generic field rate — typical layouts run 6 plugs per m² across the field and step up to 8 plugs per m² on corners and edges, with additional fixings around openings.
Handling & Storage on Site
- Keep dry and shaded: Stack boards flat on a clean, level surface. Cover with sheeting if outdoor storage exceeds a few weeks — prolonged UV softens the closed-cell surface.
- Score, don't snap: Cut with a fine-tooth saw or sharp blade rather than snapping the rebated edge. A clean cut keeps the bonding face true and protects the closed-cell surface.
- Brush the rebate before adhesive: Dust on the lap joint breaks the foam adhesive seal; a quick brush gives a full-face bond.
- Set the DPC course line first: Mark the DPC line and the facade-EPS course position before fitting the first 90 mm board. Working from a chalked datum keeps the plinth-to-facade alignment true across the whole elevation.
Certifications & Compliance — Atlas Fundament 90 mm
Atlas Fundament boards are manufactured to recognised European foundation insulation standards. Reaction to fire is classified under EN 13501-1, and the declared thermal conductivity (λ 0.038 W/mK) appears on the manufacturer's current Declaration of Performance. For projects requiring specific compressive strength values, water absorption test results, or Approved Document L 2021 calculation inputs (current at time of writing), request the latest DoP at order — exact certified values can vary between production batches and the current Atlas Fundament datasheet is the authoritative reference for compliance submissions.
Is Atlas Fundament 90 mm Right for Your Project?
- Choose this thickness if your project plans a 90 mm graphite EPS facade above DPC — the foundation board matches the facade plane and lets the basecoat, mesh, and render wrap the DPC transition without thickness compensation.
- Working at standard Part L depth? The Atlas Fundament 80 mm at R 2.11 m²K/W is the volume-runner where the facade EPS sits at 80 mm or where standard foundation U-value targets apply.
- Pushing past current Part L thresholds? The Atlas Fundament 100 mm at R 2.63 m²K/W is the specification for Passive-leaning new-builds and deep retrofit projects.
- Detailing the plinth-to-facade transition? The plinth thermal-bridge guide walks through the overlap detail that keeps the DPC line condensation-free regardless of the chosen thickness.
What to Order Next — Pack Sizes, Lead Times, Compatible Components
Atlas Fundament 90 mm is supplied as a single board (0.75 m² coverage). For project calculations, allow 1.33 boards per m² plus 5–10% cutting waste — the alignment-driven nature of 90 mm specifications means cutting waste tends to land at the lower end of the range when the plinth is set out properly. To complete the foundation build-up, order Ceresit CT84 XPS Foam for bonding and a fixing pattern of 6–8 plugs per m² above grade, stepping up to 8 plugs per m² on exposed corners and edges.
FAQ — Atlas Fundament 90 mm Coverage, Compatibility, Installation
How many 90 mm Atlas Fundament boards do I need per square metre?
Each board covers 0.75 m² (600 × 1250 mm), so 1.33 boards per m² is the base figure. For plinth runs where the board height is set against the DPC line and the facade-EPS course position, add 5–10% cutting waste for corners and openings, then round up to the nearest full board.
Why 90 mm rather than 80 or 100 mm?
The 90 mm thickness exists for one reason: alignment with the 90 mm graphite EPS facade thickness most commonly specified above DPC on UK domestic EWI projects. When the plinth board and the facade board land on the same plane, the basecoat and mesh wrap the DPC transition without thickness compensation, and the render finish reads as one continuous plane from ground level to eaves. Where the facade EPS sits at 80 mm or 100 mm, match the foundation board to that thickness instead.
What adhesive bonds Atlas Fundament 90 mm to a foundation wall?
A specialist polyurethane adhesive such as Ceresit CT84 XPS Foam is the standard route — it cures fast in cool, damp UK ground-level conditions and bonds reliably to closed-cell board surfaces. Apply in continuous beads around the perimeter plus a central bead, brush dust from the rebated edge before placement, and press firmly to spread the foam across the full bonding face.
Does 90 mm need mechanical fixings above grade?
Above ground level on plinth zones, yes — the 90 mm board's face area catches enough wind uplift that mechanical fixings are standard practice. Typical layouts run 6 plugs per m² across the field and 8 plugs per m² on exposed corners and edges, with additional fixings around openings. Below ground level, backfill provides permanent mechanical restraint and the polyurethane foam adhesive alone is typically sufficient.
Can I use 90 mm Atlas Fundament on a facade above DPC?
The closed-cell structure limits vapour permeability, so the main facade above DPC typically benefits from graphite EPS or mineral wool rather than foundation-grade board — vapour-permeable insulation lets moisture migrate outward through the render system. The 90 mm Atlas Fundament earns its place at the plinth and below-DPC zones where its moisture resistance is the deciding factor, with matching-thickness graphite EPS continuing the build-up above DPC.
How do I detail the plinth-to-facade transition with 90 mm?
Mark the DPC line and the facade-EPS course position as a single datum, then fit the 90 mm Atlas Fundament so the top of the board sits flush with the bottom edge of the first facade-EPS course above DPC. With the planes aligned, wrap the joint with mesh and basecoat as a continuous reinforced layer — the plinth thermal-bridge guide walks through the recommended overlap dimensions for the basecoat reinforcement.